Methods and apparatuses for mitigating tin whisker growth on tin and tin-plated surfaces  by doping tin with germanium

ABSTRACT

The present disclosure generally relates to the field of tin electroplating. More specifically, the present disclosure relates to methods for mitigating tin whisker formation on tin-plated films and tin-plated surfaces by doping the tin with germanium.

STATEMENT OF GOVERNMENT RIGHTS

This invention was made with U.S. Government support under ContractNumber W912HQ-10-C-0022 awarded by the U. S. Department of Defense. TheU.S. Government has certain rights in this invention.

TECHNOLOGICAL FIELD

The present disclosure generally relates to the field of tinelectroplating. More specifically, the present disclosure relates tomethods for mitigating tin whisker formation on tin-plated films andtin-plated surfaces by doping the tin with germanium.

BACKGROUND

The worldwide transition to lead-free electronics is forcing most majorsuppliers of electronic components to convert their product lines fromtin/lead-containing finishes to lead-free finishes. As a result, mostelectronics suppliers have moved to pure electroplated tin finishes.However, there is a tendency of electroplated pure tin finishes to formtin whiskers that extend a distance from the surface. Such tin whiskershave been found to form on a wide variety of tin-plated components, andunder a wide range of environmental conditions. Since these tin whiskersare comprised of nearly pure tin and are therefore electricallyconductive, they can cause problems, such as, for example, shorting ofelectronic components. Therefore the growth of tin whiskers fromtin-plated surfaces continues to cause reliability and other problemsfor electronic systems that use components that are plated with tin.Undesirable effects on electronics attributable to tin whisker formationon tin-plated surfaces have caused significant customer dissatisfactionresulting in significant financial impact on the manufacturers ofelectronics. To date, the only way to ensure that tin whiskers do notgrow within an electronic system is to eliminate pure tin from such asystem. However, the increasing reliance on the use of tin andtin-plated components in the electronic industry makes this tinelimination strategy unworkable. One tin whisker mitigation strategy hasbeen to immerse all tin-plated component leads into molten tin/lead,from the tip of the lead up to the component body. However, this processcan undesirably affect the component and is expensive to implement intothe manufacturing process.

BRIEF SUMMARY

According to one variation, the present disclosure relates to a methodfor mitigating tin whisker growth on a substrate surface. Agermanium-containing compound is dissolved to make agermanium-containing solution. Water and a complexing agent are thenadded to the germanium-containing solution. A water-solubletin-containing compound is then added to the germanium-containingsolution. An optional surfactant/leveling agent may be added before orafter the tin-containing compound is added to the germanium-containingsolution. Electrodes are immersed into the solution with the electrodesconnected to an electrical power source capable of providing anelectrical current. The power source is activated to provide theelectrical current to the solution resulting in an amount of germaniumand tin co-deposited onto the cathodic substrate surface. According toone variation, the cathodic substrate surface comprises copper, acommonly used material for electronic components such as, for example,leads. Preferably, the germanium and tin are co-deposited onto thesubstrate surface to a thickness from about 1 to about 10 microns, withan amount of from about 0.5 to about 5 weight percent by weightgermanium and 99.5 to about 95% be weight tin co-deposited on thesubstrate surface.

According to further variations, the germanium-containing compound isselected from the group including germanium dioxide, or othergermanium-containing compound that can be solubilized into aqueoussolutions, preferably alkaline solutions. Preferably, germanium dioxideis dissolved in a sodium hydroxide solution. According to a stillfurther variation, the germanium-containing compound is provided to thesolution directly as a salt, such as germanium fluoroborate, or otherwater-soluble germanium salt, and combinations thereof. It is understoodthat the tin-containing compound is added to the solution as awater-soluble salt, preferably tin (II) sulfate.

The present disclosure further relates to a method for mitigating tinwhisker growth on a substrate surface comprising the steps of,dissolving an amount of a germanium-containing compound in a basicsolution, (preferably germanium dioxide dissolved in a sodium hydroxidesolution), adding an amount of water, preferably deionized water, to thegermanium-containing compound in solution, adding a complexing agent(preferably d,l-tartaric acid), optionally adding a surfactant/levelingagent, and dissolving an amount of tin-containing compound (preferablytin (II) sulfate) into the germanium-containing solution. Atin-containing anodic electrode is immersed into thegermanium-containing and tin-containing solution and a cathodicsubstrate surface is immersed into the germanium-containing andtin-containing solution. An electrical power source is provided to theanodic electrode and the cathodic substrate (acting as an electrode)comprising a cathodic substrate surface, and then activated to providean electrical current to the electrodes, resulting in co-depositing anamount of germanium and tin onto the substrate surface. The systems,methods and apparatuses of the present disclosure could also be used andincorporated into systems and methods using a three electrode systemwith the third electrode being used as a reference electrode.

In a further variation, the present disclosure relates to a method formaking an electroplating bath comprising the steps of dissolving anamount of a germanium-containing compound in a basic solution(preferably germanium dioxide in an amount of sodium hydroxidesolution), adding an amount of water (preferably deionized water) to thegermanium-containing solution, adding an amount complexing agent(preferably d,l-tartaric acid) to the germanium-containing solution,optionally adding a surfactant/leveling agent, and dissolving an amountof water-soluble tin-containing compound (preferably tin (II) sulfate)into the germanium-containing solution. In addition, the presentdisclosure contemplates an electroplating bath made according to theabove method.

In a still further variation, the present disclosure relates to anelectroplating bath comprising an amount of a germanium-containingcompound in an aqueous solution (preferably germanium dioxide in anamount of sodium hydroxide solution), an amount of water added to thesolution, an amount of complexing agent (preferably d,l-tartaric acid),an amount of optional surfactant/leveling agent, and an amount oftin-containing compound (preferably tin (II) sulfate).

Still further, the present disclosure relates to a coating formitigating tin whisker growth by co-depositing an amount of a germaniumand tin onto a substrate surface. According to a preferred variation,the germanium and tin are electro-deposited onto a substrate surface,preferably to a thickness of from about 1 micron to about 10 microns.Preferably, the substrate surface comprises copper, and the germanium ispreferably co-deposited with the tin onto the substrate at aconcentration of from about 0.5 to about 5 weight percent germanium, andmore preferably, from about 1 to about 2 weight percent germanium.

The present disclosure contemplates the described coatings as usefullycoating any object, including, but in no way limited to, electroniccomponents where it is desirable to mitigate the formation of tinwhiskers by replacing a pure tin-containing surface with a tin andgermanium plating.

BRIEF DESCRIPTION OF THE DRAWINGS

Having thus described variations of the disclosure in general terms,reference will now be made to the accompanying drawings, which are notnecessarily drawn to scale, and wherein:

FIGS. 1 a and 1 b are flowcharts for processes of plating a coatingcomprising germanium and tin onto a substrate surface;

FIG. 2 is a schematic representation of an electroplating bath forplating a germanium and tin coating onto a substrate surface;

FIG. 3 is a flowchart for a process of plating a coating comprising puretin onto a substrate surface;

FIGS. 4 and 5 are micro-photographs of tin whiskers growing from a puretin-plated substrate surface;

FIG. 6 is a micro-photograph of a surface coated with a platingcomprising germanium and tin;

FIG. 7 is a schematic representation of an electronic component withleads oriented along the perimeter of the component body; and

FIG. 8 is a further enlarged schematic representation of the leads shownin FIG. 7.

DETAILED DESCRIPTION

The present disclosure relates to the development of electroplated tinfilms that are doped with germanium to suppress the growth of tinwhiskers from the plated substrate surface, as otherwise commonly occurswith tin-plated substrates. The addition to tin of amounts of germaniumof from about 0.5 to about 5 weight percent germanium has now been shownto significantly suppress undesired tin whisker growth.

FIG. 1 a shows a flow chart for one preferred electroplating methodvariation 10 a. An amount of germanium-containing compound was dissolvedin an aqueous solution 12 a. An amount of water was added 14 a to thegermanium-containing solution. An amount of complexing agent was added16 a to the germanium solution. Optionally, an amount ofsurfactant/leveling agent 17 a was added to the germanium solution. Anamount of water-soluble tin-containing compound was dissolved intosolution and added to the germanium solution 18 a. The tin and germaniumsolution was then used to electroplate a substrate surface 19 a.

FIG. 1 b shows a flow chart for one preferred electroplating methodvariation 10 b. An amount of germanium-dioxide was dissolved in a sodiumhydroxide solution 12 b. An amount of deionized water was added 14 b tothe germanium-containing solution. An amount of d,l-tartaric acid wasadded 16 b to the germanium solution. Optionally, an amount ofsurfactant/leveling agent 17 b was added to the germanium solution. Anamount of tin (II) sulfate was added to the germanium solution 18 b. Thetin and germanium solution was then used to electroplate a substratesurface 19 b.

As shown in FIG. 2, an electroplating bath 20 comprises container 24comprising a germanium- and tin-containing electrolyte solution 22 intowhich is suspended an anode 26 (e.g. a pure tin anode, a tin andgermanium anode, etc.) and a cathode 28 (e.g. a copper or other metalliccathode, etc.).

Example 1

GeO₂ (99.98%, Aldrich) in an amount of 0.1479 g was dissolved in 2.086 gof 1N NaOH solution (Integra Chemical). The solids were ground with aTeflon rod until the solids dissolved. An amount of 6 ml of deionizedwater was added to the solution and stirred until a substantially clearand colorless solution was achieved. An amount of 0.3919 g ofd,l-tartaric acid (99%, Alfa Aesar) was dissolved into the solution withstirring to obtain a substantially clear and colorless solution. Thesolution was then tested and found to have a pH of 2.28. Asurfactant/leveling agent, Triton X-100 (Dow Chemical), in an amount of0.0618 g in 20 ml of deionized water was added with stirring. Tin (II)sulfate (99.6%, Alfa Aesar) in an amount of 0.340 g was dissolved in theelectrolyte solution to obtain a translucent, colorless solution. Theelectrolyte solution was then used to electroplate substrate surfaces.According to the present disclosure, germanium is co-deposited with tinonto a substrate surface in the electroplating solution in a preferredamount of from about 0.5 to about 5 weight percent germanium, morepreferably, from about 1 to about 2 weight percent germanium.

Plating was conducted using 30 ml of the GeO₂/Sn electrolyte solutiondescribed immediately above at 18° C. in a 50 ml beaker with stirring.(See FIG. 2, stirring not shown.). The anode was constructed from tinsheet (99.998%, Aldrich) and had a surface area of approximately 2 cm².Two coupons were plated at a time. The two coupons were used as thecathode by connecting both of them together using an alligator clip. Thetwo coupons had a total surface area of 2 cm². Plating was conducted at0.995 volts and 14 milliamps for 8 minutes to yield a light gray matteplated film on the coupons. The tin anode was cleaned using 500 grit SiCpaper before each set of samples was plated.

The first and seventh germanium-doped tin films were analyzed byinductively coupled plasma (ICP) spectroscopy. The ICP results are shownin Table 1, along with other properties of the tin and germaniumplatings and pure tin control platings.

TABLE 1 ICP Average Analysis ICP Analysis Plating Roughness RoughnessGrain Plated (First (Last Thickness as Plated as Plated Size Grain FilmCoupon) Coupon) (microns) (Ra) (nm) (TIR) (nm) (microns) Morphology Sn4.9 105 725 2.88 Columnar SnGe 1.0% Ge 1.1% Ge 2.3-3.8 117 715 2.33Columnar

Typically, the tin and germanium plated films were completely dissolvedoff the coupons using a mixture of 8 ml of 1:1 nitric acid and 4 mls ofconcentrated hydrochloric acid in a small beaker. This solution was thentransferred to a 100 ml volumetric flask, diluted to volume withdeionized water, and analyzed to confirm the presence of the elements ofinterest (Ge and Sn) in the plating by using an ICP spectrometer. Thesurface roughness of the plating was measured using a KLA-TencorAlpha-Step 200 profilometer. The average surface roughness (Ra) and themaximum trough to peak roughness (TIR) were also measured (see Table 1).

FIG. 3 is a flowchart showing the method 30 for electroplating the puretin-coated samples for use as comparative control samples. This wasachieved using the method developed by Yun Zhang (described in U.S. Pat.No. 5,750,017). Triton X-100 (Dow Chemical) in an amount of 0.1259 g wasdissolved in 80 ml of deionized water 32. Methanesulfonic acid (70%)(Aldrich) in an amount of 20 ml was added 34. Phenolphthalein solution(0.5%) (Aldrich) in an amount of 2.00 g was added drop wise whilestirring 36. Tin methanesulfonate solution (50%) (Aldrich) in an amountof 10 ml was added to the solution while stirring 38. Plating wasconducted using 30 ml of the above electrolyte solution held at 50° C.in a 50 ml glass beaker while stirring 39. The anode was constructedfrom 99.998% tin sheet (Aldrich). Plating was performed at 0.045 V and10.9 milliamps for 8 minutes to yield a gray satin plating.

Immediately after plating, the test specimens were put into a 50° C./50%relative humidity chamber in an effort to accelerate tin whiskerformation and growth. Specimens plated with pure tin were also put intothe test chamber for use as a control. At approximately 6 months, 12months and 18 months, the test specimens were examined using a scanningelectron microscope (SEM). The pure tin plated films had numerousnodules and whiskers growing from the surface. See FIG. 4 (3500×magnification after 12,000 hours of aging) and FIG. 5 (300×magnification after 12,000 hours of aging). In contrast, thegermanium-doped tin plated films had zero whiskers develop across the 1mm² area evaluated over the same 6 month, 12 month and 18 monthevaluation periods. See FIG. 6 (1000× magnification after 12,000 hoursof aging).

As shown in the Example above, various surfactants may be added to theelectrolyte solution containing the germanium and tin. Preferredsurfactants are non-ionic surfactants that act as leveling agents tohelp obtain a substantially uniform coating when plating onto asubstrate. Preferred surfactants include Triton X-100, Igepal CA-630,Nonidet P-40, Conco NI, Dowfax 9N, Igepal CO, Makon, Neutronyx 600series, Nonipol NO, Plytergent B, Renex 600 series, Solar NO, Sterox,Serfonic N, T-DET-N, Tergitol NP, Triton N, etc., with Triton X-100being particularly preferred.

Without being bound to a particular theory, it is believed that thed,l-tartaric acid serves to complex the germanium ions and probably thetin ions in solution. In theory, two metals with different electromotivepotentials cannot be practically plated at the same time. Thislimitation is usually overcome by chemically complexing one or bothmetals, which effectively brings their electromotive potentials closertogether and allows them both to be plated/deposited at the same time.Other complexing agents that may work for the tin and germanium systeminclude without limitation, citric acid, succinic acid, aspartic acid,EDTA, mannitol, or any organic compound with carboxylic acid groups, orother groups capable of complexing metal ions in solution, etc.

The germanium-doped tin coatings affected through the processes setforth in this disclosure are understood to be deposited onto a substrateof choice to a preferred thickness of from about 1 to about 50 microns,and more preferably to a thickness of from about 1 to about 10 microns,with a preferred germanium concentration of from about 0.5 to about 5%by weight, and more preferably from about 1 to about 2 weight percent.It is understood that the germanium may be present in concentrations inexcess of 5%. However, the tin whisker mitigation observed during 18months of observation was achieved with germanium concentrations of onlyabout 1%. It is believed that excessive germanium concentrations couldimpact the economic feasibility of the disclosed methods and coatings,perhaps without offering enhanced performance relative to tin whiskermitigation. In addition, the germanium concentration must not interferewith the physical and chemical performance of the tin relative to, forexample, soldering of the coated component, etc.

FIG. 7 shows an enlarged schematic view of a representative electroniccomponent having tin-plated leads. As shown, component 70 has tin-platedcopper leads 72 about the periphery and extending from the body ofcomponent 70. FIG. 8 is a further enlargement of a cross-sectional viewof a tin-plated copper lead 72 showing the copper 74 coated by a tinelectroplate 76. It is understood that the electroplated coatings of thepresent disclosure will find utility relative to any and all electroniccomponents and parts comprising copper or other metals where a tincoating would be required to make parts solderable, for example.

The examples presented herein contemplate use of the tin and germaniumplatings on objects including electronic components such as, forexample, quad flat packs, plastic dual in-line packages (PDIPs),small-outline integrated circuits (SOICs), relays, etc., or as a platingfor traces on printed circuit boards, etc. It is further contemplatedthat such electronic parts plated with the tin and germanium coatings ofthe present disclosure will find utility in any electronics systemsused, for example, in any aircraft, spacecraft, terrestrial ornon-terrestrial vehicles, as well as stationary structures and objects.A non-exhaustive list of contemplated vehicles include manned andunmanned aircraft, spacecraft, satellites, terrestrial, non-terrestrialand surface and sub-surface water-borne vehicles, etc.

While the preferred variations and alternatives of the presentdisclosure have been illustrated and described, it will be appreciatedthat various changes and substitutions can be made therein withoutdeparting from the spirit and scope of the disclosure. Accordingly, thescope of the disclosure should only be limited by the accompanyingclaims and equivalents thereof.

We claim:
 1. A method for mitigating tin whisker growth on a substrate surface comprising the steps of: preparing a solution comprising an amount of a germanium-containing compound and a water soluble tin-containing compound and a complexing agent; immersing an anodic electrode into the solution, immersing a cathodic substrate into the solution, said cathodic substrate comprising a cathodic substrate surface; connecting the anodic electrode and the cathodic substrate to an electrical power source capable of providing an electrical current; activating the electrical power source to provide the electrical current to the anodic electrode, the cathodic substrate and the solution; and co-depositing an amount of germanium and tin onto the cathodic substrate surface.
 2. The method of claim 1, wherein the germanium and tin are co-deposited onto the substrate surface to a thickness of from about 1 to about 10 microns.
 3. The method of claim 1, wherein the germanium and tin are co-deposited onto the substrate surface at a concentration of from about 0.5 to about 5% by weight germanium.
 4. The method of claim 1, wherein the germanium-containing compound comprises a water-soluble germanium salt.
 5. The method of claim 1, wherein the germanium-containing compound is solubilized in a basic solution.
 6. The method of claim 5, wherein the germanium-containing compound is germanium dioxide solubilized in a sodium hydroxide solution.
 7. The method of claim 1 wherein the tin-containing compound is added to the solution as a water soluble tin salt.
 8. The method of claim 1, wherein the tin-containing compound is tin (II) sulfate.
 9. A method for mitigating tin whisker growth on a substrate surface comprising the steps of: dissolving an amount of germanium dioxide in a sodium hydroxide-containing solution to produce a germanium-containing solution; adding an amount of water to the germanium-containing solution; adding an amount of d,l-tartaric acid to the germanium-containing solution; dissolving an amount of tin (II) sulfate in the germanium-containing solution to produce a tin-and-germanium-containing solution; immersing a tin-containing anodic electrode into the tin-and-germanium-containing solution; immersing a cathodic substrate into the tin-and-germanium-containing solution, said cathodic substrate comprising a cathodic substrate surface; connecting an electrical power source to the anodic electrode and the cathodic substrate; activating the electrical power source to provide an electrical current to the anodic electrode and the cathodic substrate; and co-depositing an amount of germanium and tin onto the cathodic substrate surface.
 10. The method of claim 9, further comprising the step of: adding an amount of a surfactant/leveling agent to the germanium-containing solution.
 11. A method for making an electroplating bath comprising the steps of: dissolving an amount of a germanium-containing compound to make a germanium-containing solution; adding an amount of water to the germanium-containing solution; adding an amount of complexing agent to the germanium-containing solution; and dissolving an amount of water-soluble tin-containing compound into the germanium-containing solution.
 12. The method of claim 11, wherein the germanium-containing compound is selected from the group consisting of: germanium-containing compounds soluble in a basic solution, water soluble germanium salts, and combinations thereof.
 13. The method of claim 11, wherein the germanium-containing compound is germanium dioxide solubilized in a sodium hydroxide solution.
 14. The method of claim 11, wherein the complexing agent is d,l-tartaric acid.
 15. The method of claim 11, further comprising the step of adding an amount of surfactant/leveling agent to the germanium-containing solution.
 16. The method of claim 11, wherein the tin-containing compound comprises tin (II) sulfate.
 17. An electroplating bath made according to the method of claim
 11. 18. An electroplating bath comprising: an amount of a germanium-containing compound in an aqueous solution; an amount of water; an amount of a complexing agent; and an amount of water-soluble tin-containing compound.
 19. The electroplating bath of claim 18, wherein the germanium-containing compound is germanium dioxide solubilized in a basic solution.
 20. The electroplating bath of claim 18, wherein the complexing agent is d,l-tartaric acid.
 21. The electroplating bath of claim 18, further comprising an amount of surfactant/leveling agent.
 22. The electroplating bath of claim 18, wherein the tin-containing compound is tin (II) sulfate.
 23. An electroplated coating for mitigating tin whisker growth on a substrate surface comprising: a co-deposited amount of from about 0.5 to about 5 weight percent germanium and a co-deposited amount of from about 95 to about 99.5 weight percent tin.
 24. The coating of claim 23, wherein the germanium and tin are co-deposited onto the substrate surface to a thickness of from about 1 to about 10 microns.
 25. An electronic component comprising the coating of claim
 23. 26. An object comprising the coating of claim
 23. 27. An aircraft comprising the object of claim
 26. 